20 Unsafe operations

As an introduction to this section, to borrow from the official docs, "one should try to minimize the amount of unsafe code in a code base." With that in mind, let's get started! Unsafe blocks in Rust are used to bypass protections put in place by the compiler; specifically, there are four primary things that unsafe blocks are used for:

• dereferencing raw pointers
• calling a function over FFI (but this is covered in a different part of the book)
• changing types through std::cast::transmute
• inline assembly

Raw Pointers

Raw pointers * and references &T function similarly, but references are always safe because they are guaranteed to point to valid data due to the borrow checker. Dereferencing a raw pointer can only be done through an unsafe block.

// pointer.rs
fn main() {
    let raw_p: *const u32 = &10;

    unsafe {
        assert!(*raw_p == 10);
    }
}

Transmute

Allows simple conversion from one type to another, however both types must have the same size and alignment:

// transmute.rs
fn main() {
    let u: &[u8] = &[49, 50, 51];

    unsafe {
        assert!(u == std::mem::transmute::<&str, &[u8]>("123"));
    }
}